[red-knot] consider imports to be declarations (#13398)
I noticed that this pattern sometimes occurs in typeshed:
```
if ...:
from foo import bar
else:
def bar(): ...
```
If we have the rule that symbols with declarations only use declarations
for the public type, then this ends up resolving as `Unknown |
Literal[bar]`, because we didn't consider the import to be a
declaration.
I think the most straightforward thing here is to also consider imports
as declarations. The same rationale applies as for function and class
definitions: if you shadow an import, you should have to explicitly
shadow with an annotation, rather than just doing it
implicitly/accidentally.
We may also ultimately need to re-evaluate the rule that public type
considers only declarations, if there are declarations.
